Copying it is right out. By copying I mean posting the pertinent file as if it was yours without previous agreement and credit given by linking back to the original. That will result in copyright being enforced fully. This also includes alterations meant to obscure any copyright lines I may have on my work. E-mail or note me if you intend to do this so we can make an arrangement.
Derivatives will be judged on a case by case basis. As a rule of thumb, they should be different enough from the original, have artistic merit and give credit by linking back to the original. While I'm usually lenient with this I would prefer I would be made aware of these works before or as they are posted.
Referencing as long as it doesn't fall in the derivative bin is welcome though a link back to your references would be polite to the artists you referenced from. While noting or e-mailing me about it not required it's definitely a plus. If you want, you can have a critique to go along with it.
The above will be judged by me personally. While I appreciate the thought when people report art theft I'd rather when it pertains to me that it would be reported by note or e-mail without any action being taken. I'm not sure I like the actions of other people being confused for my own.
By posting the links to these works here I mean to say that these are fine by me:
If I've forgot any feel free to remind me